This invention is directed to a control circuit utilizing silicon controlled rectifiers (SCR's) for controlling the operation of a series motor powered from a direct current source.
It is well known that the direct current supplied to a load, such as a direct current motor, from a power source, such as a battery, may be selectively varied by controlling the average power supplied to the load, and that a solid state SCR can be used as a switching device to repeatedly connect and disconnect the battery to and from the load. The power supplied to the load is determined by the ratio between the time the SCR is turned on and the time the SCR is turned off.
Turning the SCR repeatedly on and off will allow a series of pulses of current to flow through the load, the frequency of the pulses being determined by the number of times the SCR is turned on per unit time and the duration of width of the pulses being determined by the length of time that the SCR remains on before it is turned off. If the pulse width remains constant during normal operation, as it is in the present invention, the ratio of on-time to off-time will vary directly as the frequency of the pulses. As the pulse frequency increases, the off-time between the time the SCR is turned off and the time it is turned back on will decrease, and more power will be delivered to the load.
Typically, SCR control systems include a main SCR which is connected in series with the load, and a pulse generator to supply a chain of gate pulses to turn the main SCR on repeatedly. A commutating capacitor is provided which will charge in a commutating direction through a charging SCR. The charging of the commutating capacitor is typically done when the main SCR is conducting. At the appropriate time a commutating SCR is turned on to connect the charged capacitor across the main SCR and divert the current therefrom so that the main SCR is reversely biased and will turn off.